Oxford phase 3 unicompartmental knee replacement in Korean patients

 KNEE

Oxford phase 3 unicompartmental knee replacement in Korean patients

H-C. Lim, J-H. Bae, S-H. Song, S-J. Kim From Guro Hospital, Korea University Medical Center, Seoul, Korea

 H-C. Lim, MD, PhD, Professor  S-H. Song, MD, Orthopaedic Surgeon  S-J. Kim, MD, Orthopaedic Surgeon Korea University Medical Center, Guro Hospital, #80 Guro-Dong, Guro-Ku, 152-050 Seoul, Korea.  J-H. Bae, MD, PhD, Professor Korea University Medical Center, Ansan Hospital, 516, Gojan-dong, Danwon-gu, 425707 Ansan, Korea. Correspondence should be sent to Dr S-J. Kim; e-mail: [email protected] ©2012 British Editorial Society of Bone and Joint Surgery doi:10.1302/0301-620X.94B8. 29372 $2.00 J Bone Joint Surg Br 2012;94-B:1071–6. Received 15 February 2012; Accepted after revision 7 March 2012

VOL. 94-B, No. 8, AUGUST 2012

Medium-term survivorship of the Oxford phase 3 unicompartmental knee replacement (UKR) has not yet been established in an Asian population. We prospectively evaluated the outcome of 400 phase 3 Oxford UKRs in 320 Korean patients with a mean age at the time of operation of 69 years (48 to 82). The mean follow-up was 5.2 years (1 to 10). Clinical and radiological assessment was carried out pre- and post-operatively. At five years, the mean Knee Society knee and functional scores had increased significantly from 56.2 (30 to 91) preoperatively to 87.2 (59 to 98) (p = 0.034) and from 59.2 (30 to 93) to 88.3 (50 to 100) (p = 0.021), respectively. The Oxford knee score increased from a mean of 25.8 (12 to 39) preoperatively to 39.8 (25 to 58) at five years (p = 0.038). The ten-year survival rate was 94% (95% confidence interval 90.1 to 98.0). A total of 14 UKRs (3.5%) required revision. The most common reason for revision was dislocation of the bearing in 12 (3%). Conversion to a total knee replacement was required in two patients who developed osteoarthritis of the lateral compartment. This is the largest published series of UKR in Korean patients. It shows that the mid-term results after a minimally invasive Oxford phase 3 UKR can yield satisfactory clinical and functional results in this group of patients.

The Oxford unicompartmental knee replacement (UKR) was developed in 1978.1,2 It was designed to simulate the normal knee by using a congruent mobile meniscal bearing and has been shown to give excellent long-term results in carefully-selected patients, not only by the designing group but also from many independent centres throughout Europe and the United States.3-5 Furthermore, it can be carried out using a minimally invasive technique that permits rapid discharge, prompt recovery and better function.6,7 We have identified few studies on the midterm results of the Oxford UKR in an Asian population.8 Kim et al8 reported that the early results were satisfactory and improved the American Knee Society (AKS) knee and function scores9 while re-establishing the range of movement of the knee. However, they assessed only 95 patients and the mean follow-up was only three years. Mullaji, Sharma and Marawar10 have shown successful results with UKR in India but the follow-up was unclear and they used both mobile and fixed-bearing implants. Furthermore, there have been concerns that UKR in the Asian population may result in a higher rate of complication and revision because of differences in lifestyle. The long-term survivorship in Asian population has not yet been established.

Therefore we investigated the clinical outcome, ten-year survival rate, and rate of complication after UKR using the Oxford phase 3 implants in a Korean population. Our hypothesis was that the results of UKR in Korea generated by an experienced surgeon who treated a large number of patients would be comparable with those achieved in the West.

Patients and Methods Between January 2001 and January 2011 we prospectively assessed all patients who underwent a medial UKR. There were 440 medial UKRs implanted in 360 patients. We were unable to assess 40 UKRs: 19 patients had died, 13 had inadequate radiographs or post-operative functional scores and eight were lost to follow-up. Therefore the final study group comprised 320 patients (400 knees): bilateral UKRs were carried out in 80 patients. The mean age of the patients at the time of operation was 69 years (48 to 82); the mean followup was 5.2 years (1 to 10). The indications for UKR were disabling knee pain with medial compartmental disease; an intact anterior cruciate (ACL) and collateral ligaments; a preoperative contracture of less than 15° and a pre-operative deformity of < 15°. Patients were excluded if they had suffered a previous tibial 1071

1072

H-C. LIM, J-H. BAE, S-H. SONG, S-J. KIM

Table I. Mean clinical outcome scores at various time intervals (OKS, Oxford knee score; KSS, Knee Society score; FS, functional score) Mean (SD) score

Pre-operative

One year

Five years

Ten years

OKS KSS FS

25.8 (8.3) 56.2 (16) 59.2 (18)

40.2 (6.5) 91.1 (12.5) 90.2 (13.5)

39.8 (7.5) 87.2 (10.4) 88.3 (14.2)

37.8 (8.4) 85.1 (18) 86.9 (14)

Table II. Life table for all 400 UKRs revised for implant-related re-operation (CI, confidence interval) Follow-up (yrs)

Number at start

Revised

Survival rate (95% CI)

0 to 1 1 to 2 2 to 3 3 to 4 4 to 5 5 to 6 6 to 7 7 to 8 8 to 9 9 to 10

400 365 328 282 244 201 166 128 99 44

3 3 3 2 0 0 2 0 1 0

99.22 (98.7 to 99.7) 98.36 (97.5 to 99.1) 97.40 (96.4 to 98.1) 96.66 (95.8 to 97.4) 96.66 (95.7 to 97.3) 96.56 (95.3 to 97.7) 96.36 (94.9 to 97.7) 95.36 (93.0 to 97.8) 94.03 (91.1 to 97.0) 94.03 (90.1 to 98.0)

100 90 80

Survival (%)

plateau fracture or had undergone an osteotomy around the knee. No patient was excluded on the basis of excessive weight. All patients gave their written informed consent. All operations were undertaken at one hospital by a single surgeon (HCL), who used the Oxford meniscal-bearing UKR (Biomet, Warsaw, Indiana). Each operation was performed through a minimally-invasive incision using a medial parapatellar approach. No ligament release was undertaken. Tibial preparation used an extramedullary jig. The tibial saw guide was applied with its shaft parallel to the long axis of the tibia in both the coronal and sagittal planes to make cut slope backwards and downwards by 7°. The depth of resection was measured using a 2 mm offset stylus, measured from the lowest point of exposed subchondral bone. The tibia was then cut both in the transverse and sagittal planes. The tibial and femoral components were cemented with Palacos cement (Stryker Orthopaedics, Mahwah, New Jersey). The anatomical bearing was used after 2006; it had not previously been available in Korea. All patients received three peri-operative doses of antibiotics and all were managed with foot pumps and stockings rather than chemical thromboprophylaxis. Post-operatively, patients began routine physiotherapy with weight-bearing as tolerated. The Knee Society score (KSS) and functional score (FS)11 and the Oxford knee score (OKS)12 were used to assess the clinical outcome. Patients were reviewed after one month, two months, six months and then at yearly intervals. Any complications were noted. Range of movement (ROM) of the knee was measured by an experienced physiotherapist (SHS) using a long-arm goniometer (arm length 50 cm; Enraf-Nonius, Delft, The Netherlands).13 Weight-bearing anteroposterior (AP) and lateral radiographs were obtained at each follow-up visit using StarPACS PiView STAR 5.0.6.0 software (Infinitt Healthcare

70 60 50 40 30 20 10 0 1

2

3

4

5

6

7

8

9

10

Follow-up (years) Fig. 1 Kaplan-Meier survival curve with 95% confidence intervals showing survival of Oxford Unicondylar knee replacement with implant-related re-operations as the endpoint.

Co., Seoul, Korea). The pre-operative and post-operative hip-knee-ankle (HKA) axes14 were measured from fulllength standing radiographs. Radiological assessment of lateral compartment arthritis was performed using the Kellgren-Lawrence (K-L) method by independent radiologists.15 Statistical analysis. For statistical analysis we used SPSS software version 12.0 (SPSS Inc., Chicago, Illinois). The differences between the mean pre-operative and post-operative clinical scores were analysed using the Student’s t-test. The chi-squared test was used to evaluate the correlation between dislocation and the size of the femoral component and to compare differences in the rates of dislocation before and after the introduction of the anatomical bearing. A p-value < 0.05 was considered statistically significant. Using revision for any cause as an end-point, a life-table was constructed, THE JOURNAL OF BONE AND JOINT SURGERY

OXFORD PHASE 3 UNICOMPARTMENTAL KNEE REPLACEMENT IN KOREAN PATIENTS

1073

Table III. Summary of the complications related to the unicompartmental knee replacement Complications

Number of cases (%) Treatment

Bearing dislocation 10 (2.5%) 2 (0.5%)

Implantation of a thicker bearing Conversion to a total knee prosthesis due to lateral compartment arthritis

Femoral component 1 (0.25%) loosening

Conversion to a total knee prosthesis

Post-operative infection

Two-stage revision with antibiotics (conversion to a total knee prosthesis)

1 (0.25%)

Table IV. Bearing dislocations related to the size of the of femoral component Femoral component sizes Extra small

Small

Number of femoral component sizes (%) Number of bearing dislocations (%)

344 (86%)

48 (12%) 8 (2%)

Medium

0 (0%)

10 (83%)

2 (17%)

0 (0%)

0 (0%)

Large

Fig. 2a

Fig. 2b

and the rates of survival determined using the life-table method. The 95% confidence intervals (CI) were calculated.

Results At final review, the mean ROM had increased from 129° (120° to 135°) pre-operatively to 133° (127° to 150°); the mean KSS and FS increased significantly from 56.2 (30 to 91) to 87.2 points (59 to 98) (p = 0.034, Student’s t-test) and 59.2 (30 to 93) to 88.3 points (50 to 100) (p = 0.021, Student’s t-test), respectively. The mean OKS increased from 25.8 (12 to 39) to 39.8 points (25 to 58) (p = 0.038, Student’s t-test). The clinical data at intervals of one, five and ten years, are summarised in Table I. Using revision for any cause as an endpoint, the cumulative survival rate at ten years was 94.03% (95% CI 90.1 to 98.0) (Table II, Fig. 1). The the mean HKA axis was 170° (162° to 178°) pre-operatively and 178° (171° to 185°) at final review. There were 14 further operations related to the UKR, an incidence of 3.5% (Table III); 12 (3%) were for dislocation; nine of these were due to a further injury, usually involving twisting the flexed knee. Of these 12 UKRs, ten were treated by implantation of a thicker bearing and two by conversion to a total knee replacement (TKR), because of osteoarthritis of the lateral compartment. There was no relationship between dislocation of the bearing and the size of the femoral component (p = 0.955, chi-squared) (Table IV). The rate of dislocation of the bearing decreased from 3.2% (seven of 220 knees) to 2.8% (five of 180 knees) after the introduction of the anatomical bearing. This difference was not statistically significant (p = 0.527, chisquared). The mean time to dislocation was 22 months (4 VOL. 94-B, No. 8, AUGUST 2012

Fig. 2c Anteroposterior (a) and lateral (b) radiographs of a 65-year-old female patient one day before re-operation for posterior dislocation of the meniscal component, and c) anteroposterior fluoroscopy during the re-operation showing the extent of posterior extra-articular migration of the meniscal component.

to 60). In one case, the bearing had dislocated posteriorly (Fig. 2). One UKR had loosening of the femoral component (Fig. 3) and one had deep infection, which was treated by two-stage revision knee procedure. There were no failures from subsidence, tibial component loosening or polyethylene wear. In six knees the arthritic change in the lateral compartment progressed from K-L grade 1 pre-operatively to K-L grade 3 and in four knees to K-L grade 4. In five knees there was post-operative valgus of > 10°: all showed progressive lateral compartment degenerative changes; two

1074


Fig. 3a

Fig. 3b

Anteroposterior radiograph of a 61-year-old woman that shows femoral component loosening five years post-operatively and b) after conversion to a total knee replacement.

Table V. Details of survival of the Oxford medial unicompartmental knee replacement Author/s

Year

Mean follow-up (yrs) (range)

Number

Survival rate (%)

Revision number (n, %)

Failure due to lateral osteoarthritis (n, %)

Bearing dislocation number (n, %)

Svard and Price17 Emerson et al23 Keys et al18 Rajasekhar et al19 Langdown et al20 Price et al22 Vorlat et al24 Pandit et al4 Current series

2001 2002 2004 2004 2005 2005 2006 2011 2012

12.5 (10.1 to 15.6) 6.8 (2 to 13) 7.5 (6 to 10) 5.8 (2 to 12) 5.2 (1 to 13) 15 5.5 (1 to 10) 5.6 (1 to 11) 5.2 (1 to 10)

124 50 40 135 29 439 149 1000 400

95 93 100 94 100 93.1 84 96 94

6 (4.8) 7 (14) 0 (0) 5 (3.7) 0 (0) 23 (5) 24 (16) 29 (2.9) 14 (3.5)

0 (0) 4 (8) 0 (0) 0 (0) 0 (0) 7 (1.5) 9 (6) 9 (0.9) 2 (0.5)

3 (2.4) 0 (0) 0 (0) 2 (1.4) 0 (0) 5 (1.1) 4 (2.6) 6 (0.6) 12 (3)

were converted to a TKR. Patellofemoral degenerative changes progressed in eight asymptomatic patients. In addition, four patients with limited flexion post-operatively required manipulation of the knee under anaesthesia. Two patients (two UKRs) had a superficial infection and two had a deep-vein thrombosis (DVT). There were no other complications.

Discussion The Oxford phase 3 UKR, which became available in 1998, came in a larger range of sizes than previous versions, as well as with instrumentation designed so that the procedure could be performed through a short skin incision without eversion of the patella.6 In Europe, Price et al6 reported that the mean recovery time after minimally-invasive Oxford phase 3 UKR was half that of the phase 2 UKR and one-

third that of a TKR. In 2006, Pandit et al16 reported that the survival rate was 97.3% (95% CI 92.0 to 100.0) at seven years. We have performed a prospective study of the Oxford phase 3 UKR in Korean patients and found high functional scores and a satisfactory ROM at a mean of 5.2 years. This study also showed a ten-year survival rate of 94% (95% CI 90.1 to 98.0) that is equivalent to the results of a recent large-scale European study4 and others (Table V).17-20 Even though there was concern that implanting the Oxford knee through a limited exposure might compromise limb alignment and clinical results, we found this not to be the case in Asian patients. There are relatively few complications of UKR but their cause and treatment remain controversial. They include polyethylene wear and breakage, aseptic loosening, dislocation of the spacer, contralateral osteoarthritis, infection, THE JOURNAL OF BONE AND JOINT SURGERY

OXFORD PHASE 3 UNICOMPARTMENTAL KNEE REPLACEMENT IN KOREAN PATIENTS

tibial plateau fracture, limited ROM and unexplained pain.21,22 Disease progression in the lateral compartment is most common cause of revision4,22,24,25 and may occur if the knee is overcorrected.23 In our study, only two (0.6%) underwent conversion to a TKR because of osteoarthritis of the lateral compartment. This is probably due to the fact that the overall post-operative alignment was slightly varus. Previous studies have reported failure of UKR if the knee is overcorrected into valgus.26,27 Most papers advocate relative undercorrection of the alignment with the presumption that overcorrection increases the risk of progressive degenerative change in the remaining compartment.28-30 Kennedy and White31 reported that the best results can be obtained when the mechanical axis runs through the centre of knee joint or slightly medial to it. Scott et al32 reported that overcorrection increases the risk of degenerative change in the remaining compartment. In our series, all five knees that were in > 10° of valgus had progressive degenerative changes in the lateral compartment. However, this could simply be due to the natural progression of the underlying arthritic disease.25 There was a relatively higher number of dislocations (3%) in our series compared with others (Table V). Pandit et al4 recently reported a dislocation rate of 0.6% and Price et al22 a rate of 1.1%. The reason for this might be inadequate stability of the bearing in high flexion, which happens more frequently in Asian patients.33 In a Western population, the proportion of patients whose knees flex more than 120° is not high and is not required by their lifestyle.34,35 This is different in Asia, where full flexion from squatting and sitting on the floor may strain the anterior cruciate ligament and be a causative factor of bearing dislocation.33 Previously, Kim et al8 have reported a dislocation rate of 2.1% and Choy et al33 a rate of 9.1% in the Korean population. As high-flexion is essential in Asian cultures,10,36 where for social and religious reasons people sit on the floor, care must be taken to prevent dislocation of the bearing. When this occurs, it is usually anteriorly directed and is rarely found in other directions.37,38 Nevertheless, we identified one knee with incarceration of a dislocated bearing posteriorly following injury that required a separate posterior incision to retrieve it (Fig. 2). Chan et al39 reported that patients who had a one-stage bilateral UKR had a significantly higher rate of proximal DVT than those who had a two-stage procedure. In our study, two patients who underwent unilateral UKR had a DVT. They both had underlying cardiovascular disease and were > 70 years of age. In Asia, the use of prophylactic agents against DVT remains controversial.40,41 Patients in our series did not receive routine prophylactic anticoagulants and were managed with foot pumps and stockings. In conclusion, this independent prospective study showed a high rate of survival of the Oxford Phase 3 UKR in the medium-term. We believe this large series gives a relatively accurate assessment of the outcome in Korean patients. Excellent, durable, and reliable results can be VOL. 94-B, No. 8, AUGUST 2012

1075

expected for this procedure in the Asian population when strict inclusion criteria are followed. However, dislocation of the bearing occurs more frequently than in Western populations because of the social and religious demands placed on the function of the prosthesis. This study was supported by a grant of the Korea Healthcare technology R&D Project, Ministry of Health & Welfare, Republic of Korea (A110416). No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.

References 1. Goodfellow J, O’Connor J. The mechanics of the knee and prosthesis design. J Bone Joint Surg [Br] 1978;60-B:358–369. 2. Goodfellow J, O’Connor J, Murray DW. The Oxford meniscal unicompartmental knee. J Knee Surg 2002;15:240–246. 3. Khanna G, Levy BA. Oxford unicompartmental knee replacement: literature review. Orthopedics 2007;30-5(Suppl):11–14. 4. Pandit H, Jenkins C, Gill HS, et al. Minimally invasive Oxford phase 3 unicompartmental knee replacement: results of 1000 cases. J Bone Joint Surg [Br] 2011;93B:198–204. 5. Lisowski LA, van den Bekerom MP, Pilot P, van Dijk CN, Lisowski AE. Oxford Phase 3 unicompartmental knee arthroplasty: medium-term results of a minimally invasive surgical procedure. Knee Surg Sports Traumatol Arthrosc 2011;19:277–284. 6. Price AJ, Webb J, Topf H, et al. Rapid recovery after oxford unicompartmental arthroplasty through a short incision. J Arthroplasty 2001;16:970–976. 7. Muller PE, Pellengahr C, Witt M, et al. Influence of minimally invasive surgery on implant positioning and the functional outcome for medial unicompartmental knee arthroplasty. J Arthroplasty 2004;19:296–301. 8. Kim KT, Lee S, Park HS, Cho KH, Kim KS. A prospective analysis of Oxford phase 3 unicompartmental knee arthroplasty. Orthopedics 2007;30(Suppl):15–18. 9. Liow RY, Walker K, Wajid MA, Bedi G, Lennox CM. The reliability of the American Knee Society Score. Acta Orthop Scand 2000;71:603–608. 10. Mullaji AB, Sharma A, Marawar S. Unicompartmental knee arthroplasty: functional recovery and radiographic results with a minimally invasive technique. J Arthroplasty 2007;22(Suppl):7–11. 11. Ewald FC. The Knee Society total knee arthroplasty roentgenographic evaluation and scoring system. Clin Orthop Relat Res 1989;248:9–12. 12. Dawson J, Fitzpatrick R, Murray D, Carr A. Questionnaire on the perceptions of patients about total knee replacement. J Bone Joint Surg [Br] 1998;80-B:63–69. 13. Lenssen AF, van Dam EM, Crijns YH, Verhey M, et al. Reproducibility of goniometric measurement of the knee in the in-hospital phase following total knee arthroplasty. BMC Musculoskelet Disord 2007;8:83. 14. Cooke TD, Scudamore RA, Bryant JT, et al. A quantitative approach to radiography of the lower limb: principles and applications. J Bone Joint Surg [Br] 1991;73B:715–720. 15. Kellgren JH, Lawrence JS. Radiological assessment of rheumatoid arthritis. Ann Rheum Dis 1957;16:485–493. 16. Pandit H, Jenkins C, Barker K, Dodd CA, Murray DW. The Oxford medial unicompartmental knee replacement using a minimally-invasive approach. J Bone Joint Surg [Br] 2006;88-B:54–60. 17. Svard UC, Price AJ. Oxford medial unicompartmental knee arthroplasty: a survival analysis of an independent series. J Bone Joint Surg [Br] 2001;83-B:191–194. 18. Keys GW, Ul-Abiddin Z, Toh EM. Analysis of first forty Oxford medial unicompartmental knee replacement from a small district hospital in UK. Knee 2004;11:375–377. 19. Rajasekhar C, Das S, Smith A. Unicompartmental knee arthroplasty. 2- to 12-year results in a community hospital. J Bone Joint Surg [Br] 2004;86-B:983–985. 20. Langdown AJ, Pandit H, Price AJ, et al. Oxford medial unicompartmental arthroplasty for focal spontaneous osteonecrosis of the knee. Acta Orthop 2005;76:688– 692. 21. Vardi G, Strover AE. Early complications of unicompartmental knee replacement: the Droitwich experience. Knee 2004;11:389–394. 22. Price AJ, Waite JC, Svard U. Long-term clinical results of the medial Oxford unicompartmental knee arthroplasty. Clin Orthop Relat Res 2005;435:171–180. 23. Emerson RH Jr, Hansborough T, Reitman RD, Rosenfeldt W, Higgins LL. Comparison of a mobile with a fixed-bearing unicompartmental knee implant. Clin Orthop Relat Res 2002;404:62–70. 24. Vorlat P, Putzeys G, Cottenie D, et al. The Oxford unicompartmental knee prosthesis: an independent 10-year survival analysis. Knee Surg Sports Traumatol Arthrosc 2006;14:40–45.

1076


25. Emerson RH Jr, Higgins LL. Unicompartmental knee arthroplasty with the Oxford prosthesis in patients with medial compartment arthritis. J Bone Joint Surg [Am] 2008;90-A:118–122. 26. Laskin RS. Unicompartmental tibiofemoral resurfacing arthroplasty. J Bone Joint Surg [Am] 1978;60-A:182–185. 27. Murray DW, Goodfellow JW, O'Connor JJ. The Oxford medial unicompartmental arthroplasty: a ten-year survival study. J Bone Joint Surg [Br] 1998;80-B:983–989. 28. Cartier P, Sanouiller JL, Grelsamer RP. Unicompartmental knee arthroplasty surgery: 10-year minimum follow-up period. J Arthroplasty 1996;11:782–788. 29. Heck DA, Marmor L, Gibson A, Rougraff BT. Unicompartmental knee arthroplasty: a multicenter investigation with long-term follow-up evaluation. Clin Orthop Relat Res 1993;286:154–159. 30. Schai PA, Suh JT, Thornhill TS, Scott RD. Unicompartmental knee arthroplasty in middle-aged patients: a 2- to 6-year follow-up evaluation. J Arthroplasty 1998;13:365–372. 31. Kennedy WR, White RP. Unicompartmental arthroplasty of the knee: postoperative alignment and its influence on overall results. Clin Orthop Relat Res 1987;221:278– 285. 32. Scott RD, Cobb AG, McQueary FG, Thornhill TS. Unicompartmental knee arthroplasty: eight- to 12-year follow-up evaluation with survivorship analysis. Clin Orthop Relat Res 1991;271:96–100. 33. Choy WS, Kim KJ, Lee SK, et al. Medial unicompartmental knee arthroplasty in patients with spontaneous osteonecrosis of the knee. Clin Orthop Surg 2011;3:279–284.

34. Laubenthal KN, Smidt GL, Kettelkamp DB. A quantitative analysis of knee motion during activities of daily living. Phys Ther 1972;52:34–43. 35. Noble PC, Gordon MJ, Weiss JM, et al. Does total knee replacement restore normal knee function? Clin Orthop Relat Res 2005;431:157–165. 36. Koshino T, Saito T, Orito K, et al. Increase in range of knee motion to obtain floor sitting after high tibial osteotomy for osteoarthritis. Knee 2002;9:189–196. 37. Robinson BJ, Rees JL, Price AJ, et al. Dislocation of the bearing of the Oxford lateral unicompartmental arthroplasty: a radiological assessment. J Bone Joint Surg [Br] 2002;84-B:653–657. 38. Weale AE, Feikes J, Prothero D, et al. In vitro evaluation of the resistance to dislocation of a meniscal-bearing total knee prosthesis between 30 degrees and 90 degrees of knee flexion. J Arthroplasty 2002;17:475–483. 39. Chan WC, Musonda P, Cooper AS, et al. One-stage versus two-stage bilateral unicompartmental knee replacement: a comparison of immediate post-operative complications. J Bone Joint Surg [Br] 2009;91-B:1305–1309. 40. Fong YK, Ruban P, Yeo SJ, et al. Use of low molecular weight heparin for prevention of deep vein thrombosis in total knee arthroplasty: a study of its efficacy in an Asian population. Ann Acad Med Singapore 2000;29:439–441. 41. Ansari S, Warwick D, Ackroyd CE, Newman JH. Incidence of fatal pulmonary embolism after 1,390 knee arthroplasties without routine prophylactic anticoagulation, except in high-risk cases. J Arthroplasty 1997;12:599–602.

THE JOURNAL OF BONE AND JOINT SURGERY